This invention relates to an improved low excess air burner having a movable venturi in the burner throat, air inlets distributed generally about the entire circumference of the burner, and having air flow distributing means in the air inlets. The movable venturi, air inlet and air flow distributing means act to create a smooth air flow and an even air distribution across the burner throat opening.
The movable venturi shaped insert is adjustable through a predetermined range of positions to control the flame shape and length at high loads and to trim the air flow at low loads. The burner housing has been provided with a construction in the form of an annular insert forming a hollow cylinder along a middle section and expanding frustoconically forwardly and rearwardly to the size of the burner housing in a front section and a rear section respectively.
Tapered throats in burner housings have been known in the art for some time. An example of a burner having a tapered throat is discussed in Reid et al, U.S. Pat. No. 3,940,234 which utilizes a tapered throat construction 58 at the input end of the burner tube 24 which provides a narrowing throat portion which narrows to a diameter 29, has a front expanding portion 33, and a rear expanding portion which does not increase to the size of the burner housing thus providing a step or offset at the position where air is admitted to the burner housing. Due to the step at this position, the air and high velocity gas are mixed turbulently in the venturi portion of the burner tube and turbulent flow is maintained throughout the burner housing. The venturi is constructed to cause and maintain turbulent flow, not to maintain a smooth, evenly distributed flow throughout the length of the burner tube as in the present invention. Furthermore, the Reid Patent does not contemplate moving the venturi to adjust the flame front. For different burn rates and varying conditions, the optimum position for the venturi will vary. To maintain a smooth, evenly distributed flow throughout the burner housing as contemplated by the applicant, it is necessary to move the venturi to maintain a smooth, evenly distributed flow, to trim the flame shape and length at high loads and to trim the air flow at low loads.
It has been known in the art to slide a sleeve over air openings admitting air to the burner throat to restrict air flow therethrough and to move such sleeves with rods attached thereto which extend rearwardly through the rear wall of the burner, as shown by Peterson et al, U.S. Pat. No. 3,904,349. The Peterson patent discloses the positioning rods 64 which position the sleeve 60 and the positioning rod 64A which positions the sleeve 82.
A drawback to the fuel burners known in the art is that the burners have been located where the air around the air inlet is irregular in pressure and velocity and therefore an irregular flow of air is admitted through the air inlet to the burner housing. Applicant has determined that it is desirable to admit a smooth, evenly distributed supply of air to the burner and thus there is a need to provide air inlets with an inlet housing and perforated plate shield to insure that a smooth, evenly distributed air supply enters the burner housing.